NEW YORK (AP) — A new study suggests that monarch butterflies use an internal magnetic compass to help navigate on their annual migrations from North America to central Mexico.

Scientists already knew they navigate by the sun. But the insects do just fine on very cloudy days, leading to suspicions they also use a magnetic compass, like migratory birds and sea turtles. Previous studies haven’t made a clear case for that, according to researchers who report the new work Tuesday in the journal Nature Communications.

The scientists tethered monarchs in a chamber without any outdoor light and showed that their flight patterns responded to changes in the magnetic field. Further work suggested the compass is in the antennae. Millions of the black-and-orange butterflies spend the winter in Mexico.